Nearly all residential plumbing systems, as well as most commercial plumbing systems, include at least one water heater. Various types of water heater are available, depending on the size and type of plumbing system, on the expected patterns of usage, and on the local costs for various forms of energy.
Many water heating systems include a water storage tank in which heated water is stored until use. Such “tank-based” water heaters are adaptable to almost any form of energy, including oil, gas, and electricity, and can provide high quantities of heated water on demand.
However, tank-based water heaters suffer from several disadvantages. They are generally required to maintain large quantities of water at elevated temperatures for extended periods of time between use, and this wastes energy and money. Tank-based water heaters are typically large, difficult to site, and expensive, so that even very large plumbing systems usually include only a few, or typically only one, tank-based water heater, which is often located in a basement or other non-central location. This means that hot water must be piped over significant distances so as to serve all desired locations in a plumbing system, thereby wasting more energy and money, and requiring users to run the water for several seconds or minutes before hot water arrives.
In addition, a tank-based water heater has a finite water capacity, and can require considerable time to replenish its hot water reservoir once it is exhausted. This can be a major inconvenience for someone who wishes to bathe or otherwise use hot water after the capacity of a tank-based water heater has been exhausted. The situation is even worse if the user is in the process of bathing when the hot water runs out.
Another approach that is popular in many locations throughout the world is to use a tankless water heater that can heat water on demand as it flows through the system. Some tankless water heaters use oil or gas to heat water, but these systems suffer from many of the same disadvantages as tank-based water heaters. They are typically large, difficult to site, and expensive, so that even very large plumbing systems usually include only a few, or typically only one, oil or gas heated tankless water heater, typically located in a basement or other non-central location. As with tank-based water heaters, this means that hot water must be piped over significant distances so as to serve all desired locations in a plumbing system, thereby wasting energy and money and requiring users to wait for arrival of hot water.
Electrically heated tankless water heaters largely avoid the problems cited above. They are relatively small and inexpensive, thereby allowing a plurality of tankless water heaters to be distributed throughout a plumbing system, typically being sited at or near the locations where hot water is used. However, electricity is generally more costly than oil or gas, so it is critical that an electric, tankless water heater use energy as efficiently as possible.
Some electric tankless water heaters include an electric heating element surrounding a pipe or a small chamber through which water flows. The heating element is typically surrounded by thermal insulation in an attempt to minimize heat loss. Nevertheless, at least some of the heat generated by the heating element is inevitably lost to the environment without passing through the water, thereby wasting energy and increasing the operating cost.
In other approaches, the electric heating element is contained within a small flow-through heating chamber, the heating element being typically folded back onto itself within the heating chamber so as to simplify the electrical wiring by placing both ends of the heating element at the same end of the chamber. However, this approach can waste energy due to uneven exposure of different regions of the heating element to the surrounding water.
Some electric tankless water heaters are designed to create turbulence in the water as it flows through the heating chamber, while others are designed to promote laminar water flow. Electric tankless water heaters that are designed to encourage turbulence within the heating chamber provide good thermal contact between all of the water and the heating element, and this improves the energy efficiency. However, the turbulence creates added back-pressure in the plumbing system, which must be overcome by enlarging the cross-sectional flow-through area of the heating chamber, leading to larger water heaters.
Electric tankless water heaters that encourage laminar water flow through the heating chamber provide lower back-pressures, but are generally less energy efficient, because only some of the water is brought into direct contact with the heating element. These units therefore require larger heating elements and correspondingly larger heating chambers.
Furthermore, many electric tankless water heaters, although small, are complex in design and expensive to produce, thereby limiting the number of units that can be deployed in a plumbing system without exceeding the available budget.